A base mod for Source 2013 which expands and improves the toolset available to modders, augmenting other maps and mods for development purposes. It contains new logic entities, new I/O and keyvalues, a custom implementation of VScript, custom shaders, countless fixes, and enhanced FGDs all in one single package. Mappers would create their maps based off of this mod and larger mods with their own unique changes would draw from Mapbase's code and assets.
A comparison in between Source 2013's original projected textures and Mapbase's projected textures, at least when it comes to shaders.
You can find more information on projected textures in Mapbase on the wiki article:
Github.com